The present invention relates generally to a process for the production of a layer composition by treatment of electrically conductive polymers with stabilizers, layer compositions and the use of such layer compositions.
Electrically conductive polymers from the class of polypyrroles, polyanilines and polythiophenes are known from the literature. Poly(3,4-alkylenedioxythiophene) dispersions in particular have recently acquired technical importance, since they can be used, for example, for the production of conductive or antistatic coatings (see e.g. EP-A 440 957). In practice, however, it has been found that the electrical conductivity of the coatings from such dispersions is not always sufficiently stable for practical uses at a higher temperature.
EP 1 798 259 A1 and WO 2008/055834 A1 describe that the heat stability of the electrical conductivity of coatings containing polythiophene dispersions can be increased if, for example, derivatives of gallic acid (3,4,5-trihydroxybenzoic acid) are added. Nevertheless, the addition of these stabilizers often leads to the dispersions used becoming unstable. Furthermore, these stabilizers cannot be added to reactive mixtures of monomers and oxidizing agents for the production of conductive coatings, since these stabilizers interfere in the oxidative polymerization, e.g. by complexing with the oxidizing agent.
There therefore continued to be a need for processes for the production of electrically conductive layers from conductive polymers with improved temperature stability and layer compositions comprising these.
One of the main fields of use for electrically conductive layer compositions comprising conductive polymers is solid electrolyte capacitors.
A commercially available electrolyte capacitor as a rule is made of a porous metal electrode, an oxide layer on the metal surface, an electrically conductive material, usually a solid, which is introduced into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation. An electrolyte capacitor which is frequently used is the tantalum electrolyte capacitor, the anode electrode of which is made of the valve metal tantalum, on which a uniform, dielectric layer of tantalum pentoxide has been generated by anodic oxidation (also called “forming”). A liquid or solid electrolyte forms the cathode of the capacitor. Aluminium capacitors in which the anode electrode is made of the valve metal aluminium, on which a uniform, electrically insulating aluminium oxide layer is generated as the dielectric by anodic oxidation, are furthermore frequently employed. Here also, a liquid electrolyte or a solid electrolyte forms the cathode of the capacitor. The aluminium capacitors are usually constructed as wound- or stack-type capacitors.
π-conjugated polymers are particularly suitable as solid electrolytes in the capacitors described above because of their high electrical conductivity. π-conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals with respect to processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene used technically being poly(3,4-ethylenedioxythiophene) (PEDOT), since it has a very high conductivity in its oxidized form.
The solid electrolytes based on conductive polymers can be applied to the oxide layer in various ways and manners. EP-A-0 340 512 thus describes, for example, the preparation of a solid electrolyte from 3,4-ethylenedioxythiophene and the use thereof in electrolyte capacitors. According to the teaching of this publication, 3,4-ethylene-dioxythiophene is polymerized on to the oxide layer in situ.
The disadvantage of the production of solid electrolyte capacitors using an in situ polymerization is however, amongst others, the complexity of the process. Thus, a polymerization process which in each case comprises the process steps of impregnation, polymerization and washing as a rule lasts several hours. Under certain circumstances, explosive or toxic solvents must also be employed here. A further disadvantage of the in situ process for the production of solid electrolyte capacitors is that as a rule anions of the oxidizing agent or, where appropriate, other monomeric anions serve as counter-ions for the conductive polymer. Because of their small size, however, these are not bonded to the polymer in a sufficiently stable manner. As a result, diffusion of the counter-ions and therefore an increase in the equivalent series resistance (ESR) of the capacitor may occur, especially at elevated use temperatures of the capacitor. The alternative use of high molecular weight polymeric counter-ions in the chemical in situ polymerization does not lead to sufficiently conductive films and therefore does not lead to low ESR values.
Alternative processes for the preparation of solid electrolytes based on conductive polymers in electrolyte capacitors have therefore been developed in the prior art. Thus, for example, DE-A-10 2005 043828 describes a process for the production of solid electrolytes in capacitors, in which a dispersion comprising the already polymerized thiophene, for example the PEDOT/PSS dispersions known from the prior art, is applied to the oxide layer and the dispersing agent is then removed by evaporation.
Generally, the object according to the invention was to eliminate or at least mitigate the disadvantages emerging from the prior art.
The object of the present invention was furthermore to provide layer compositions which can be produced easily with conductive polymers and display good properties in capacitors and other uses, such as antistatic layer compositions. Furthermore, a process for the production of these layer compositions which can readily be used commercially, especially in the abovementioned uses, is to be provided.
A further object was to improve the heat stability of such layer compositions, in particular as polymer solid electrolyte capacitors.
Furthermore, an object according to the invention was to provide a layer composition which, in particular as a capacitor, displays advantageous properties, such as an equivalent series resistance (ESR) which increases as little as possible and a dissipation factor which likewise increases just as little, with a capacitance which is as constant as possible. In the case of layers, a lowest possible increase in the surface resistance was to be achieved.
A contribution towards achieving at least one of the abovementioned objects is made by a process for the production of a layer composition with an electrically conductive layer, comprising the process steps:                a) provision of a substrate with a substrate surface;        b) formation of a polymer layer comprising an electrically conductive polymer on at least a part of the substrate surface;        c) application of a liquid stabilizer phase, comprising a stabilizer and a liquid phase, to the polymer layer from process step b), wherein the stabilizer phase contains less than 0.2 wt. %, based on the stabilizer phase, of the electrically conductive polymer,        
wherein the stabilizer is an aromatic compound having at least two, preferably at least three, further preferably at least four and furthermore preferably at least six OH groups.
It has been found, completely surprisingly, that the application of stabilizers to an electrically conductive polymer layer containing conductive polymers, in particular in capacitors, leads to a significant improvement in the heat stability of the layer or of the capacitor.